Polystyrene is a very desirable insulating material due to ease of use and low cost. It is noted however that polystyrene is also an extremely flammable material when melted. This can sometimes be an issue in buildings utilizing polystyrene insulation where a building fire might have sufficient heat to melt the polystyrene insulation thereby allowing that same material to flow through any potential openings within the building or through the roof structure feeding the fire. In addition, the loss of the EPS material to the building interior leaves the roof structure uninsulated and subject to rising temperatures. Temperatures reaching 1300° Fahrenheit as the metal structural component causes structural failure. The faster the structure attains this temperature the less time is available for emergency work before building collapse. Conversely, the longer that temperature can be staved off, the longer the emergency services personnel have to do what they do. For this reason many roof systems are specified with, and installers tend to use, polyisocyanurate insulation. This type of insulation is more expensive however, and in some cases more difficult to use than expanded polystyrene. Therefore, expanded polystyrene insulation is preferred if it is possible to use safely. Many roof structures available in the market place do not provide for the use of polystyrene insulation. In inexpensive and rapidly built roof structures, one generally cannot utilize expanded polystyrene insulation unless installed in an encapsulation of lightweight concrete which would require a stronger structure to carry the weight and would facilitate liquefaction and delivery of the lightweight concrete. Therefore, the art will be benefited by a method and construction allowing rapid roof construction the use of polystyrene insulation while maintaining fire retardancy with respect to polystyrene in low sloped commercial building structures.